Copying machines and multi-function peripherals having copy function, facsimile function, and scanner function (hereinafter, these are defined as an image forming apparatus) are widespread. The image forming apparatus, using a finishing function, can make prints such as weekly magazines, catalogues, and books (hereinafter, these are defined as booklet-shaped prints).
Book-like prints are bound so as to prevent sheets from separation, so that the bound portion in the neighborhood of the center of the two spread pages is not seen, thus the images printed on the overall two spread pages are interrupted and misshaped between the pages, and if there is a sentence at the center of the two spread pages, a problem arises that illegible letters occur, and the problem stands out, as the number of pages of book-like prints increases.
For the aforementioned problem, there is an art of shifting outwards drawing information such as letters and images from the bound portion on each of the two spread pages. For example, in Unexamined Japanese Patent Application Publication No. H07-287428, in an image forming apparatus for reading and recording on the recording sheets sequentially image data of each page of document image data composed of several pages so that the recording sheets are arranged in the order of pages when folding the recording sheets, a configuration is disclosed that a means for moving symmetrically the image recording position on each recording sheet at a predetermined distance around the folding line, a means for inputting information in accordance with the thickness of the recording sheets, and a means for changing sequentially the predetermined distance whenever recording an image on each recording sheet are installed.
Further, in Unexamined Japanese Patent Application Publication No. 2004-66582, in an image forming apparatus having an image forming means for forming images in consideration of the finishing process of sheet members, a configuration is disclosed that a sheet count detection means for detecting the number of sheets of the sheet members to be finished, a binding margin adjustment means for adjusting the binding margin at the time of image formation by the image forming means, and a mode selection means for selecting a mode selectively from a plurality of finishing modes are installed, and the image forming means, according to the number of sheets detected by the sheet count detection means and the finishing mode selected by the mode selection means, adjusts the binding margin by the binding margin adjustment means, thereby forms images.
By the methods described in Unexamined Japanese Patent Application Publication No. 7-287428 and Unexamined Japanese Patent Application Publication No. 2004-66582 which are mentioned above, as shown in FIG. 14, the visibility and appearance of the drawing information can be improved in case when both ends of the two spread pages are pulled outside and the pages can be spread largely.
However, in a case when careful handling is required like a borrowed magazine or book or if a binding margin is large and the pages can be hardly spread, as shown in FIG. 15(a), the pages on the side where there are more sheets of booklet-shaped prints 50 (the left pages in the drawing) are curved at a large radius of curvature (that is, curved slowly) or as shown in FIG. 15(b), the pages on the side where there are less sheets (the right pages in the drawing) are curved at a large radius of curvature, thus the images and letters in the neighborhood of the bound portion cannot be seen, and a problem arises that the visibility of the pages on the curved side is deteriorated.
Further, in a method for shifting outward from the bound portion, images and letters which are printed on each page, a blank portion 51 where nothing is printed is formed in the neighborhood of the bound portion, so that the blank portion 51 of each page on the side being not curved is conspicuous and a problem arises that the appearance does not look good.
The present invention was developed with the foregoing problems in view and its main object is to provide an image forming apparatus capable of improving the visibility and appearance when either of the two spread pages is curved at a large radius of curvature, an imposition method for booklet-shaped prints, and booklet-shaped prints.